1. Field of the Invention
This invention relates generally to solid state polymerization of post consumer and non-post consumer polyethylene terephthalate (PET), and more particularly to the polymerization of polyethylene terephthalate (PET) flakes in connection with the manufacture of plastic strapping.
2. Description of the Related Act
Post consumer PET, whose primary source is from plastic soft drink bottles, can be obtained from material recovery facilities. It is used to form, for example, fiberfill for insulating material in clothing and fiber material used to produce carpeting. It is of relatively low and heterogeneous intrinsic viscosity (IV). In the past, this characteristic has prevented PET from being used directly to produce products which require high or homogeneous IV. It is one of the discoveries of this invention that the heterogeneity of the IV of PET does not adversely affect strap production.
In the prior art, the PET obtained, whether post consumer or not, was chopped into flakes and chunks which were then re-extruded into pellets. The chopped PET has a relatively wide range of IV because the various soft drink bottles are made from different IVs, typically being on the order of 0.65-0.80 dl/g. The prior art has consistently taught that in order to make a high performance product from this post consumer PET material it is necessary to have a narrow range of IV material after the solid state processing which required, as an initial step, to pelletize the flakes before solid state polymerization. In order to raise and achieve a narrow range of IV of the PET-pellets, the prior art teaches to submit the PET pellets to solid state polymerization. The pellets once solid state polymerized, are then used in the production of new, higher performance products such as strapping.
The prior art begins the solid state polymerization (SSP) process with pellets of uniform geometry. The prior art SSP process, using pellets for strap production, took approximately 12 to 19 hours to complete and it was not appreciated that a heterogeneous mixture of flakes and chunk like material could be solid stated directly to the same average IV as pellets significantly faster and on the order of about 1/4 the time as for pellets. Additionally, as noted, it was believed that flakes with a wide IV range could not be solid stated directly without first being pelletized to a state suitable for production of high performance strap, suitable meaning not only of a high enough average IV but also having a narrow range of IV. The solid state polymerization of pellets was intended to result in, and did in fact result in, a narrow IV range which those skilled in the art believed was necessary in order to make a high performance product. The resulting prior art strap did not have an average IV greater than 0.90 dl/g. In contrast, it has been discovered that, contrary to the prior art teachings, high performance plastic strap having an average IV of greater than 0.90 dl/g can be made commercially in an economical manner using a PET having a wide distribution of IV after being solid stated directly from a flaked state without pelletization.